1. Field of the Invention
This invention relates to a welded joint having an excellent fatigue strength which is mainly used for welded structures such as ships, offshore structures, bridges, building machines, and so forth. More particularly, the present invention relates to a welded joint having an improved fatigue strength by increasing the area ratio of a ferrite micro-structure capable of retarding a propagation speed of a fatigue crack in a weld heat affected zone (hereinafter abbreviated as the "HAZ") of a welded joint.
2. Description of the Related Art
As the sizes of buildings have become greater, a reduction in the weight of structural members has become an important subject in recent years, and the strength of steels used for such structures has become higher and higher for accomplishing this object. However, since a load is repeatedly applied to the welded joint during use in ships, offshore structures, bridges, building machines, and so forth, counter-measures must be taken to prevent fatigue destruction in such structures. It is at the welded joint portion that this fatigue destruction is most likely to develop. Therefore, an improvement in the fatigue strength of the welded joint has been required.
Intensive studies have been made to this date on the factors deciding the fatigue strength of the welded joint and an improvement of the fatigue strength. However, most of the proposals include an improvement by additional execution after welding such as grinding by a grinder, shaping the end portion of the weld bead final layer by heating and refusing, shot peening to generate compressive stress at the weld end portion (Japanese Unexamined Patent Publication (Kokai) No. 59-110490 and No. 1-301823), and so forth. The reduction of the residual stress by post heat-treatment after welding is also well known in the art.
A method which improves the fatigue strength of the weld portion even in the as-welded state by means of components of the steel material without using a specific execution or post heat-treatment after welding as described above has also been proposed.
In order to make the formation of clean polygonal ferrite advantageous by an addition of Si, to strengthen the steel by an addition of B and to improve hardenability to thereby obtain an excellent elongation flange property, fatigue characteristics and resistance weldability, Japanese Unexamined Patent Publication (Kokai) No. 3-264645 discloses a high strength steel sheet having excellent elongation flange property, etc., which contains 0.01 to 0.2% of C, 0.6 to 2.5% of Mn, 0.02 to 1.5% of Si and 0.0005 to 0.1% of B.
Japanese Examined Patent Publication (Kokoku) No. 3-56301 discloses a ultra-low carbon steel sheet having excellent spot weldability which contains not greater than 0.006% of C, not greater than 0.5% of Mn,. not greater than 0.05% of Al and 0.001 to 0.100% of at least one of Ti and/or Nb, in total, with the proviso that nitrides and sulfides are not calculated, so as to advantageously improve the joint fatigue strength of the spot welded portion by elaborating the steel components, especially by an addition of B, and the proportion of un-recrystallized structure in the steel.
Japanese Unexamined Patent Publication (Kokai) No. 6-207245 discloses a multi-layered steel plate having excellent fatigue characteristics by means of an addition of Ni to a steel material surface layer to generate a residual stress of compression at a weld end portion so as to increase the time until the occurrence of a fatigue crack, and wherein the addition amount of Ni is at least 3% inside the region spaced apart by at least 0.2 mm from both surfaces of the steel plate and being not greater than 25% of the thickness.
Japanese Unexamined Patent Publication (Kokai) No. 6-228707 discloses a structural steel having excellent weld joint fatigue characteristics, and a welding method therefor, which contains 0.001 to 0.01% of C, 0.005 to 0.05% of Si and 0.5 to 2% of Cu and having a Ceq of not greater than 0.2. This reference invention makes uniform the hardness distribution in the proximity of the weld end portion by using fine precipitation of Cu while reducing Ceq to prevent concentration of plastic deformation, and reduces the residual stress of the weld end portion functioning as a mean stress by eliminating HAZ hardening through the reduction of Ceq.
Among these prior art references, Japanese Unexamined Patent Publication (Kokai) No. 59-110490 and Japanese Unexamined Patent Publication (Kokai) No. 1-301823 require a specific execution after welding, and cannot improve the fatigue strength in the as-welded state. The method using the heat-treatment after welding is not preferable, either, because the number of the process steps increases and execution of welding becomes complicated. Further, its effect is limited.
The thin steel sheet described in Japanese Unexamined Patent Publication (Kokai) No. 3-264645 relates to a base material of steel sheet which is mainly directed to wheels and discs of automobiles. Since this steel sheet is entirely different in application, the thickness and the method of use from the steel plate used for ship-building, offshore structures, etc., to which the present invention is directed, the observation described in this reference cannot be applied as such to the thick steel plate. Further, since this reference does not at all describe the welded joint, the influences on the fatigue strength of the welded joint are not at all examined. Further, it is not clear whether or not the polygonal ferrite micro-structure, which is reportedly contained in the base metal, is generated in the HAZ.
The steel sheet disclosed in Japanese Examined Patent Publication (Kokoku) No. 3-56301 relates to the spot weld portion of a ultra-low carbon steel sheet, and contemplates to control the hardness distribution of the spot weld portion. Spot welding is one of the resistance welding methods which clamps the weld portion of a steel sheet under pressure by electrodes, and causes a large current to flow through the weld portion within a short time. In contrast, the welding method of the welded joint as the object of the present invention is mainly a welding method used for welding steel plates having a large thickness. Therefore, these welding methods are different from each other not only in the welding residual stress but also in the welding conditions such as the shape of the electrodes, the existence of a welding material, and so forth. Because the factors governing the fatigue strength are different between spot welding of a thin sheet and welding of a thick steel plate, the observation of spot welding cannot be applied as such to the welding method of a thick steel plate.
The steel plate described in Japanese Unexamined Patent Publication (Kokai) No. 6-207245 has the same application because it is a structural steel. However, since this reference limits to a Ni-containing multi-layered steel, the fatigue strength cannot be improved in the ordinary single-layered steel. It is not obvious from this reference whether or not the fatigue strength of the welded joint can be improved.
The invention disclosed in Japanese Unexamined Patent Publication (Kokai) No. 6-228707 does not describe the HAZ structure of the weld joint, and the relation between the micro-structure and the fatigue strength is not clear, either. Therefore, this reference invention is different from the present invention. Further, the C addition amount and the Si addition amount are as small as not greater than 0.01% and not greater than 0.05%, respectively. Since the reference invention essentially requires the addition of Cu, it is different from the present invention.